Cleaner production of the lightweight insulating composites: Microstructure, pore network and thermal conductivity

Highlights

• Use geopolymer to improve the sustainability and insulating properties of lightweight composites.

• Design the pores network and the microstructure approximated by a spatial periodic geometry.

• Correlation porosity–density–thermal conductivity of lightweight composites.

Abstract

Inorganic polymer cement paste was used as cleaner binder for the design of lightweight matrices as insulating envelopes and panels in building and construction industries. Sponge-like structure with a homogeneously distributed pore network, low density and low thermal conductivity permitted to classify the geopolymer–wood fiber composites promising clean insulating materials. Matrices with the density ∼0.79 g/cm³, bi-axial four-point flexural strength of ∼4 MPa presented thermal conductivity between 0.2 and 0.3 W/(m K). The possibility of substituting the sodium silicate with rice ash-NaOH system and the efficiency of the matrices to constitute an effective tortuous road for the thermal gradient improved the sustainability and quality of this new class of products. The pores network and the microstructure approximated by a spatial periodic geometry suggested a “macro transport” mechanism to explain the movement of heat across the matrix of light geopolymer composite.

Introduction

The conventional Ordinary Portland Cement (OPC) is the most common binder generally used to design low-cost lightweight insulating materials in the areas of building and construction [1], [2], [3]. However, apart from its high CO₂ released during the production process, the detailed investigations show that these materials possess poor aptitude when applying as fire-resistant materials [4]. In fact, One of the challenges in the design of mass cement-based materials is to avoid the generation of cracks, which is caused by the heterogeneous distribution of temperature and stress [1]. Thermal gradients in cement-based materials can induce internal stress which leads to cracking on a microscopic or macroscopic scale [1], [4]. This explain the inefficiency of the OPC cement based insulating materials primarily due to their chemical composition and behavior in the context of variation of the temperature. The knowledge of the thermal conductivity and other thermal transport properties of construction materials involved in the process of heat transfer are essential in predicting the temperature profile and heat flow through the material [5], [6].

Many authors have demonstrated the positive impact of the amorphous or disordered aluminosilicates as fly ash, silica fume, perlite, metakaolin, etc. on the transport of the lightweight concretes [2], [3].

The porous and amorphous structure of inorganic polymer cements (IPC), often indicated as geopolymers, implies that flow in a thermal gradient will take a very tortuous route consisting of a multiple of neighboring interconnected polysialate particles [4], [7]. This is enhanced by the nanoporous nature of IPC and their thermal stability range that reach 800–900 °C [8]. The difference in the thermal conductivity between the OPC (1.5 W/(m K)) [7] and the IPC (0.6 W/(m K)) [4], [6] is linked to the chemical bulk composition, the densification, pore network and microstructure of the both matrices.

Kamseu et al. [4], [6] demonstrated how the reaction that implies the corrosion of alumina can be used to increase the insulating behavior of the IPC. It was possible to decrease the thermal conductivity of the dense IPC matrix down to 0.15 W/(m K) [4], [6]. The authors also noted the possible and significant impact of the pores coalescence into the heat transfer with the consequence on the efficiency for thermal insulation: decrease in mechanical strength and increase in permeability and thermal diffusivity.

The challenge regarding the mechanical properties, the pores coalescence and the thermal diffusivity were joined together to address the increasing demand for more environmentally friendly and sustainable materials and the desire to propose a useful recycling possibility to the natural wood fibers (saw dusts) from the wood industries in Cameroon and tropical areas motivated the authors of this work. The current work marks a significant advancement into the research of suitable eo-sustainable materials with thermos-regulator properties. These properties include heat and humid climate management between the 〈in-door and out-door〉 flux of energy through building walls in tropical regions, thereby improving human thermal conditions in buildings. According to the principle of the Kyoto Pyramid, the most cost effective method of reducing energy usage is to provide better thermally insulated buildings.

A node aim of this work is to promote building energy conservation through the construction of green buildings. Wood fibers reinforced with geopolymer binders are evidenced here as products of the clean production linked to the increase of the efficiency of resource utilization rate, wastes recycling, reduce and avoid the generation of pollutants, protect and improve environment, among others. Cleaner production simply means the continuous application of measures for design, improvement, and reduction of energy consumption, use of by-products and sustainable raw materials. The specific objective is to combine structural and thermal conductivity behavior to propose functional materials capable to isolate efficiently while withstands the mechanical and environmental solicitations (stresses).